Chronic renal disease is linked to prolonged bleeding, with evidence that the blood platelet does not function normally. Although the ability of platelets to aggregate is generally inhibited, some reports have failed to confirm this. One problem lies in experimental design and the use of potentially insensitive, optical aggregometry. Chronic renal disease is also associated with high levels of plasma parathyroid hormone, especially its degradation fragments. There is important recent evidence that parathyroid hormone (PTH) and some specific fragments directly inhibit platelet aggregation. However, the biochemical basis for this inhibition is not clear, although alteration of calcium and cyclic AMP metabolism is implicated. There exists the possibility that the bleeding tendencies of chronic renal failure are partially due to defective platelet function caused by elevated levels of plasma parathyroid peptides. The proposed research will therefore examine platelet function and bleeding time in subjects with chronic renal failure in relation to plasma PTH, platelet cyclic AMP and calcium. We will also investigate the mechanisms by which PTH and specific PTH fragments block in-vitro platelet function and alter normal calcium and cyclic AMP metabolism. Platelet rich plasma from subjects with chronic renal failure will be tested by new quench-flow techniques which measure platelet aggregation efficiencies by following the disappearance of single particles. Initial kinetics during the first 5 sec of aggregation will be analyzed at varying inducer concentrations. Sensitive radioimmunoassays will be used to determine the levels of specific fragments of PTH. New, second-generation fluorescent probes of internal platelet calcium (Indo-1), plus radioimmunosassays of cyclic AMP, will allow correlations of PTH, platelet function and these important regulators. For the in vitro studies of PTH and PTH peptides, (32P)-labeled, normal platelets will be employed to determine whether the hydrolysis of the phosphatidylinositols is blocked during thrombin-induced platelet activation.